1. Field of the Invention
The present invention relates to a signal reading circuit for linear image sensors such as charge coupled devices, which are connected together in a line to efficiently read the document of large size as well as the document of small size.
2. Background of the Invention
The width of reading by a linear image sensor, such as a charge coupled device, is usually about 2 to 3 cm. That width of reading is too short for the linear image sensor to read a document of size B5 or A4. For example, as shown in FIG. 3, the reading width 3 of a document 2 is larger than the length of a linear image sensor 1. In order to read the document 2 by the linear image sensor 1, a reduced image of the document 2 is focused on the photoelements of the linear image sensor 1 through a lens unit 4 and then read. For that reason, there is a disadvantage that the optical path length for making the reduced image on the linear image sensor and the size of a reading apparatus are both large.
In order to eliminate the disadvantage, a plurality of linear image sensors 1A-1D, as shown in FIG. 4 are disposed in different reading positions so that the total length of the linear image sensors is equal to the reading width of the document. Since the photoresponse properties of the linear image sensors 1A-1D need to be set equal to each other, an opaque metalization mask 5 is provided on at least the first photoelement of each of the linear image sensors 1A-1D and the levels of the output signals of the opaque metalized light-intercepted portions of the sensors are made equal to each other to equalize the photoresponse properties of the sensors 1A-1D to each other. As a result, each of the linear image sensors 1A-1D sends out a normalizing signal S1, shown in FIG. 5, having a dark reference level for the linear image sensor pixel corresponding to the opaque metalization mask 5, and an effective image signal S2 for the remaining pixels. If the output signals of the linear image sensors 1A-1D are simply synthesized together, the document 2 cannot exactly be read since the masked pixels do not provide effective image signals.
For that reason, a linear image sensor unit 10 as shown by a dot and dash line in FIG. 1 has been developed. In the linear image sensor unit 10, five linear image sensors 11-15 are disposed so that their reading positions are different from each other not only in a main scanning direction but also in sub-scanning direction perpendicular to the principal linear image sensor direction, and opaque metalized portions 11M-15M of the linear image sensors 11-15 are overlapped with the rear portions of the preceding linear image sensors 11-15. The linear image sensor unit 10 sends out, as shown in FIG. 6, a signal S1 having a dark reference level corresponding to one of opaque metalization masks 11M-15M, an effective image signal S2, and a signal S3 corresponding to the overlap of the adjacent linear image sensors. It is a problem how to process the output signals of the linear image sensor unit 10.